The present invention relates generally to technique including a method and a structure for forming substrates using a layer transfer technique. More particularly, the present method and system provides a method and system using a linear accelerator process for the manufacture of thick free standing semiconductor films for a variety of applications including photovoltaic cells. But it will be recognized that the invention has a wider range of applicability; it can also be applied to other types of applications such as for three-dimensional packaging of integrated semiconductor devices, photonic or optoelectronic devices, piezoelectronic devices, flat panel displays, microelectromechanical systems (“MEMS”), nano-technology structures, sensors, actuators, integrated circuits, biological and biomedical devices, and the like.
Alternative embodiments in accordance with the present invention relate generally to techniques including methods and apparatuses for cleaving free standing films from material in bulk form, such as a single crystal silicon ingot. Such free standing films are useful as a photovoltaic material such as a solar cell. But, it will be recognized that embodiments of the invention have a wider range of applicability; it can also be applied to other types of applications such as for three-dimensional packaging of integrated semiconductor devices, photonic devices, piezoelectronic devices, flat panel displays, microelectromechanical systems (“MEMS”), nano-technology structures, sensors, actuators, integrated circuits, biological and biomedical devices, and the like.
From the beginning of time, human beings have relied upon the “sun” to derive almost all useful forms of energy. Such energy comes from petroleum, radiant, wood, and various forms of thermal energy. As merely an example, human beings have relied heavily upon petroleum sources such as coal and gas for much of their needs. Unfortunately, such petroleum sources have become depleted and have lead to other problems. As a replacement, in part, solar energy has been proposed to reduce our reliance on petroleum sources. As merely an example, solar energy can be derived from “solar cells” commonly made of silicon.
The silicon solar cell generates electrical power when exposed to solar radiation from the sun. The radiation interacts with atoms of the silicon and forms electrons and holes that migrate to p-doped and n-doped regions in the silicon body and create voltage differentials and an electric current between the doped regions. Depending upon the application, solar cells have been integrated with concentrating elements to improve efficiency. As an example, solar radiation accumulates and focuses using concentrating elements that direct such radiation to one or more portions of active photovoltaic materials. Although effective, these solar cells still have many limitations.
As merely an example, solar cells rely upon starting materials such as silicon. Such silicon is often made using either polysilicon (i.e. polycrystalline silicon) and/or single crystal silicon materials. These materials are often difficult to manufacture. Polysilicon cells are often formed by manufacturing polysilicon plates. Although these plates may be formed effectively in a cost effective manner, they do not possess optimum properties for highly effective solar cells. In particular, polysilicon plates do not exhibit the highest possible efficiency in capturing solar energy and converting the captured solar energy into usable electrical power.
By contrast, single crystal silicon (c-Si) has suitable properties for high grade solar cells. Such single crystal silicon is, however, expensive to manufacture and is also difficult to use for solar applications in an efficient and cost effective manner.
Generally, thin-film solar cells are less expensive by using less silicon material but their amorphous or polycrystalline structure are less efficient than the more expensive bulk silicon cells made from single-crystal silicon substrates. These and other limitations can be found throughout the present specification and more particularly below.
Additionally, both polysilicon and single-crystal silicon materials suffer from material losses during conventional manufacturing called “kerf loss”, where the sawing process eliminates as much as 40% and even up to 60% of the starting material from a cast or grown boule and singulate the material into a wafer form factor. This is a highly inefficient method of preparing thin polysilicon or single-crystal silicon plates for solar cell use.
In particular, conventional techniques for manufacturing single crystal silicon substrates for incorporation into solar cells, typically involves the physical separation of thin single crystal silicon layers from a single crystal silicon ingot originally grown. One example of such a conventional manufacturing technique is inner diameter (ID) sawing.
The ID sawing technique employs a circular saw having a blade located on its inner diameter. The ingot is pushed through the center of the saw until a desired wafer thickness is on the other side of the saw. With the saw rotating, the saw is then raised or lowered to allow the blade to slice through the ingot. The ID sawing method offers a number of possible disadvantages. One is that the saw must be of minimum thickness to be sufficiently strong to withstand the stress of the sawing action. However, an amount of silicon material corresponding to this saw thickness (the kerf) is lost by this cutting. Use of even the thinnest saw blade that can reliably be used to saw the ingot, may result in losses of expensive, pure single crystal silicon to the kerf. For example, a typical saw blade kerf has a width of 300 μm, where an individual sliced wafer may have a width of only 800 μm. Use of the conventional wafer sawing technique can thus result in kerf losses of expensive, pure starting material amounting to as high as 60% of the entire ingot. Another disadvantage of the conventional ID sawing technique is that slices can only be separated one at a time, thus limiting throughput and elevating cost.
Partly in response to the limited throughput of sawing, the alternative conventional technique of wire sawing has been developed. In wire sawing, a network of rapidly moving parallel wires is provided. The side of an ingot is then contacted with the moving wires in an environment including oil and abrasives, resulting in simultaneous slicing of the wafer into a plurality of wafers. The advantages of this technique over ID sawing includes parallel sawing of the boule and producing thinner wafers of 180-250 um with a more modest 190-250 um kerf loss. While effective, conventional wire sawing also offers disadvantages, in particular a still significant kerf loss of about 50% attributable to the thickness of the wire, and possible contamination by exposure of the substrate to the oil and abrasives.
From the above, it is seen that techniques for forming suitable substrate materials of high quality and low cost are highly desired. Cost-effective and efficient techniques for the manufacture of single crystal silicon substrates for incorporation into solar cells, are also desirable.